The patent document 1 shown below discloses an exhaust gas purifying apparatus for an internal combustion engine having a lean NOx catalyst in the exhaust system. The lean NOx catalyst traps NOx and reduces the trapped NOx. Since NOx is trapped in the lean NOx catalyst during a lean operation wherein the air-fuel ratio is set to a comparatively large value, it is necessary to timely supply reducing components (HC, CO) to the lean NOx catalyst so as to reduce the trapped NOx. In the apparatus shown in the patent document 1, an amount of NOx trapped in the lean NOx catalyst is estimated according to an engine rotational speed and a demand torque of the engine. The NOx amount in the feed gas (gas discharged from the combustion chamber of the engine) changes depending on an exhaust gas recirculation amount (an amount of exhaust gases recirculated to the intake system). Therefore, in the apparatus of the patent document 1, the amount of trapped NOx is corrected according to the exhaust gas recirculation amount.
The patent document 2 shown below discloses a NOx removing apparatus which removes NOx by the selective catalytic reduction (SCR) using urea. In this apparatus, a NOx emission amount is calculated using a neural network, and a supply amount of urea liquid is adjusted according to the calculated NOx emission amount. An intake pressure, an intake pipe temperature, a fuel consumption rate, etc. are used as input parameters of the neural network and the output parameter of the neural network is an amount of NOx emitted from the engine.
[Patent Document 1] Japanese Patent Laid Open No. 2006-214322
[Patent Document 2] Japanese Patent Laid Open No. 2003-328732